U.S. Pat. No. 4,382,938 suggests a process for the preparation of zolpidem by reacting (6-methyl-2-p-tolyl-imidazo[1,2-a]pyridin-3-yl)acetic acid (zolpidic acid) with dimethyl amine in the presence of carbonyldiimidazole. Large scale production with carbonyldiimidazole is not viable as carbonyldiimidazole is very expensive, toxic, allergenic and hygrocopic.
Additional methods for making various imidazopyridines are disclosed by Schmitt et. al, Aust. J. Chem., 1997, 50, 719-725. Among them is the reaction of certain 2-phenylimidazo[1,2-a]pyridines with freshly distilled ethyl glyoxylate to form ethyl 2-hydroxy-2-(2′-phenylimidazo[1,2-a]pyridin-3′-yl)acetate. This compound is reduced by adding phosphorus tetraiodide in dichloromethyl to form ethyl 2-(2′-phenylimidazo[1,2-a]pyridin-3′-yl)acetate.
EP1038875 describes the use of ethyl glyoxylate is not convenient for a commercial scale production. Also phosphorus tetraiodide is expensive, not readily available and produces iodine- and phosphorus-containing wastes. The '875 patent describes a method of preparing imidazopyridines of formula (I)
by reacting 6-methyl-2-(4-methylphenyl)imidazo[1,2-a]pyridine of formula (II) with glyoxylic acid or a compound of formula (V)

Where R3 and R4 each independently represent hydrogen or a lower alkyl to form a compound of formula (VI)
and further removing the alpha hydroxyl group of (VI) with a hydrogenolysis agent in the presence of a hydrogenolysis catalyst to form (I). This method involves the use of expensive hydrogenolysis catalyst selected from the group consisting of palladium, platinum and rhodium. The use of formic acid, phosphonic acid and phosphinic acid as hydrogenolysis agent do not make the process of EP1038875 environment friendly.
The synthetic route for the preparation of compound of formula (I) is disclosed in EP1172364, which involves reacting a 2-phenyl-imidazo[1,2-a]pyridine of formula (II) with an oxalate, followed by reduction.
WO0214306 describes a method of preparing imidazopyridines of general formula (VI)
in which: Y denotes H, halogen or C1-4 alkyl group, X1 and X2 denote, independently of each other, hydrogen, a halogen or a C1-4 alkoxy, C1-6 alkyl, CF3, CH3S, CH3SO2 or NO2 group and R1 and R2 denote independently of each other, H or a C1-5 alkyl group, with the proviso that R1 and R2 do not both denote hydrogen or salts thereof by reacting compound of general formula (VIII)
with a compound of general formula (IX)
in which A denotes a halogen and B denotes a halogen, a C1-4 alkoxy or an NR1R2 to form a compound of general formula (X).

If B denotes a halogen or a C1-4 alkoxy, the compound of formula (X) is reacted with NHR1R2 to form compound of formula (VI).
The present invention relates to a more efficient and commercially viable process for preparing compounds of formula (I).